How Do the Phases of the Moon Affect the Tides?

You may have heard about how the Moon generates the tides. You may even know that high spring tides occur during a full or new Moon, and that low neap tides occur during a quarter Moon (when the Moon appears as a semicircle). But how does it really work? Nothing is changing about the Moon during these times except how much of it is lit up, right? Let’s find out!

Problem: Where do spring and neap tides come from?

Materials

Procedure

Cut out a Sun, Earth, and Moon from your construction paper. Cut an oval out of blue construction paper, then trim off the two sides to make a circle. Save the crescents you trimmed off of the oval—you’ll use these as the spring tides. Cut out even smaller slivers to represent neap tides. If you want, you can cut out continents in green construction paper and glue them on the Earth. Draw a dotted line through the middle to represent the equator.

The Moon is about 1/4th the diameter of the Earth. In reality, the Sun is many times larger than the Earth, but since it’s far away and for the purposes of this experiment, just cut out a circle about as big as the Moon.

Put the Sun, Earth and Moon on a table to represent the solar system. The Sun is far away (much farther than you’d be able to show on a table), so put it at one end of the table. Place the Earth so that if you continued the line of the equator, you would run into the Sun. The Moon orbits the Earth, so place it next to the Earth.

Now we will create a spring tide. First, figure out where the Moon is when it’s full. Hint: sunlight is what illuminates the face of the Moon and makes it appear as a full circle from the Earth. In which position must the Moon be in relation to the Sun and Earth to make this happen?

Now, place one of the spring tide crescent trimmings on the side of the Earth facing the moon.

There are spring tides on the other side, too, so place the other piece of “tide” on the side of the Earth opposite the Moon. What do you notice about the orientation of the Sun, Moon, and Earth during the spring tide?

Move the Moon so that it’s in a position where it will be “new,” or completely dark when viewed from Earth. What do you notice about the orientation of the Sun, Moon, and Earth?

Now, where is the Moon oriented in relation to the Sun and Earth when it is a quarter? Place your neap tide crescents with one facing the Moon, and the other on the opposite side. Remember that a “quarter moon” is when the Moon looks like a semicircle from Earth. We call this a quarter moon because only 1/4th of the Moon is illuminated (don’t forget the half of the Moon that you can’t see!).

Compare the positions of the Sun, Moon, and Earth during spring tides and neap tides. Why might this affect the tides?

Results

A full moon is when the Moon is opposite of the Earth, relative to the Sun. The new moon is when the Moon is on the same side of the Earth as the Sun. Quarter moons are on either side of the Earth, 90 degrees from the position of full or new moons. Spring tides only happen when the Moon, Earth, and Sun are lined up.

Why?

Phases of the Moon happen because of the way that the Sun lights it up. A new moon occurs when the Moon is directly between the Earth and the Sun. A full moon occurs when the Moon is directly opposite of (180 degrees from) this position. A quarter moons occurs when the Earth, Moon, and Sun form a 90 degree angle. Crescent and gibbous moons occur between these phases.

Both the Moon and the Sun exert gravitational force on Earth’s oceans. During spring tides, since the Moon and the Sun are aligned, they exert gravitational force on the tides at the same time and in the same direction. However, when the Sun and Moon are at right angles to each other during quarter moons, the forces of gravity partially cancel each other out. The Sun’s gravity is much, much stronger than the Moon’s, but it’s farther away, and the difference between one side of the Earth and the other is relatively small, too. This causes neap tides.

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